1. Technical Field
This invention relates to a method for production of a heat-resistant container, the method preparing a heat-resistant container of resin by blow molding.
2. Background Art
Containers prepared by blow molding using polyethylene terephthalate (PET) as a resin are known. Containers made of PET are excellent in transparency, toughness, sanitary aspects, etc., and are used as containers of various contents. Currently, they find widespread use, particularly, as containers containing liquid materials such as beverages. Recently, their applications spread further, and wide-mouthed containers containing semisolid materials such as jam and pasta sauce are beginning to appear. A heat-resistant container, one of the PET containers, can be filled with any of such foods and beverages heated to high temperatures for sterilization.
This type of container is filled with a material which will become contents and which have been heated to a high temperature (e.g., about 90° C.) for heat sterilization. The container with the contents is sealed with a lid, and then cooled. During this cooling, the interior of the bottle has an atmosphere at reduced pressure, as the volume of the contents decreases, or the volume of a gas remaining in a space on the mouth side (head space) formed above the contents decreases. At this time, a barrel section of the bottle may be deformed under reduced pressure within the bottle. This is undesirable in terms of appearance. With the container used under high-temperature filling conditions, therefore, deformation due to reduced pressure inside the container needs to be accommodated. For this purpose, it is common practice to form a plurality of flexible panels in the barrel section of the container. To increase rigidity and suppress deformation, moreover, use is made of a structure having three-dimensional portions (also called ribs) provided in a wall section of the container (see, for example, Patent Documents 1 and 2).
By such means as providing the vacuum panel portions in the wall section of the heat-resistant container, deformation of the heat-resistant container is suppressed, even if the interior is brought into a reduced-pressure atmosphere because of the reduced volume of the contents. This avoids a situation where the appearance deteriorates to lower the commercial value of the heat-resistant container.
Provision of such panels or ribs, however, requires the formation of many irregularities in the container. Hence, the shape of the mold becomes complicated, relatively large amounts of material are needed at the time of molding, and thus costs are high. The container itself is inevitably heavy, contrary to the recent demand for the weight reduction of containers.
If the vacuum panel portions or three-dimensional portions are provided in the wall section, a space for the disposition of a product label is limited, so that restrictions are imposed on the design of the label and the contents of its indications. If the vacuum panel portions, for example, are provided in the heat-resistant container, moreover, stepped parts are created in the wall surface. If the container is applied, for example, as a wide-mouthed heat-resistant container to be filled with a food such as jam or mayonnaise, there may be a part, to which a spoon or the like does not reach, in a range where the spoon or the like scoops the contents, and the contents may fail to be used up.
Particularly when a heat-resistant container is applied as a wide-mouthed food container which is filled with a semisolid food, therefore, a measure against deformation due to a reduced-pressure atmosphere is indispensable. Nonetheless, the wall surface still needs to be kept as flat as possible for increased commercial value.